System and method for improving bit-loading in discrete multitone-based digital subscriber line modems

ABSTRACT

A system and method for improving bit-loading in discrete multitone (DMT)-based digital subscriber line (DSL) modems. In one embodiment, the system includes: (1) a model generator configured to generate a model containing a calculated total bit loading for an assumed gross coding gain and estimated total bit loadings for a other plurality of assumed gross coding gains and (2) a bit loader associated with the model generator and configured to load bits in accordance with the model.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to digital communicationsand, more specifically, to a system and method for improving bit-loadingin discrete multitone (DMT)-based Digital Subscriber Line (DSL) modems.

BACKGROUND OF THE INVENTION

DSL is a popular way to gain broadband access to the Internet. DSL'sdistinct advantage over its competitors is its ability to provide suchbroadband access using an ordinary, twisted-pair copper telephone line,commonly referred to as a “local loop.” DSL calls for a DSL modem to belocated at a user's “premises” and a corresponding DSL AccessMultiplexer (DSLAM) at the central office (CO) that serves the user.

DSL works by dividing the total frequency spectrum available over thetelephone line into two bands: a narrow lower band consisting offrequencies of 25 kHz and below and a broad upper band consisting offrequencies above 25 kHz. The lower band is retained for conventionaltelephone use; the upper band provides the broadband access. DMT DSLcalls for the upper band to be divided into a large number of narrowbandsubchannels, on the order of a few hundred subchannels to be morespecific. Some of the subchannels are designated for downstream data andothers for upstream data. If the number of downstream subchannels equalsthe number of upstream subchannels, the DSL is said to be symmetric;otherwise, it is said to be asymmetric.

DMT was selected for DSL due to its ability to react to interference.Interference occurs on a telephone line when an external event (such aslightning) creates static in the line or the signals being transmittedover the line interfere with themselves. Interference is rarelyfull-spectrum; instead it is often limited to a range of frequencies.While interference may impair some of the DMT subchannels, many if notmost subchannels are likely to remain unaffected and available to carrysignals.

DMT DSL requires that all of its subchannels be monitored for quality(typically expressed in terms of signal-to-noise ratio, or SNR). Then,an algorithm is used to allocate bits to be transmitted among thesubchannels as a function of relative subchannel quality. The resultingallocation (or “loading”) scheme, taking into account anyuser-configurable preferences, is referred to as “bit loading.” Bitloading calls for more bits to be allocated to a DMT subchannel that hasa higher SNR than it does to one that has a lower SNR. The DSL modem,which is at the user's premises, is responsible for measuring the SNR ofthe DMT subchannels, determining the bit loading and transmitting thebits accordingly. DMT DSL that dynamically changes subchannel loading iscalled “adaptive,” because bit transmission adapts to changingsubchannel conditions.

Modern DSL framing parameters include the number of bits loaded on eachDMT subchannel, Reed-Solomon coding redundancy and codeword size,whether trellis coding is enabled or disabled, interleaver depth andoverhead subchannel rate. The user can configure the maximum delay, theminimum overhead rate, the maximum and minimum data rate (includingspecifying “best possible”), the minimum impulse noise protection, thenominal and maximum PSD level and the allowable subchannel set.

Unfortunately, no straightforward way exists to select the modem framingparameters to optimize globally for all input parameters. This problemis conventionally solved through an algorithm that searches over avariety of input parameter settings. For example, the algorithm mightsearch over trellis code enabled and disabled, varying Reed-Solomonredundancy values, and a range of gross coding gains.

Subchannel coding gain is the effective increase in subchannel gain thatresults purely from the coding scheme applied to information transmittedover a given subchannel; gross coding gain is the sum of all subchannelcoding gains. the higher the gross coding gain, the more bits can beloaded on each DMT subchannel. However, higher coding gains typicallyrequire more coding redundancy to achieve which may or may not result ina higher overall data rate (or more impulse noise protection or a higheror lower delay depending on the particular values of theuser-configurable parameters).

What is needed in the art is a way to estimate modem framing parametersglobally for at least a subset of input parameters so that the overallnet data rate of a DMT DSL modem is increased. More specifically, whatis needed in the art is a way to estimate modem framing parametersglobally for all input parameters such that errors in estimation arereduced, and most advantageously minimized, over a wide range ofoperating conditions.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention introduces a way to estimate the number of bits loadedonto each DMT subchannel as the gross coding gain changes. The onlyexact way to estimate this is to run an algorithm known as “bit loading”over all subchannels. Because bit loading is computationally intensive,the present invention provides a way to run bit loading only once atsome initial gross coding gain but maintains an accurate estimate of thebit loading over a range of gross coding gains.

In one aspect, the present invention provides a system for improvingbit-loading in DMT-based DSL modems. In one embodiment, the systemincludes: (1) a model generator configured to generate a modelcontaining a calculated total bit loading for an assumed gross codinggain and estimated total bit loadings for a other plurality of assumedgross coding gains and (2) a bit loader associated with the modelgenerator and configured to load bits in accordance with the model. Forpurposes of the present invention, an “occupied subchannel” is asubchannel to which is allocated at least one bit.

In another aspect, the present invention provides a method of improvingbit-loading in a DMT-based DSL modem. In one embodiment, the methodincludes: (1) generating a model containing a calculated total bitloading for an assumed gross coding gain and estimated total bitloadings for a other plurality of assumed gross coding gains and (2)loading bits in accordance with the model.

In yet another aspect, the present invention provides a system forbit-loading in a DMT-based DSL modem. In one embodiment, the systemincludes: (1) a model generator configured to generate a modelcontaining a calculated total bit loading for an assumed gross codinggain and estimated numbers of one-bit subchannels for a other pluralityof assumed gross coding gains and (2) a bit loader associated with themodel generator and configured to load bits in accordance with themodel.

In still another aspect, the present invention provides a method ofbit-loading in a DMT-based DSL modem. In one embodiment, the systemincludes: (1) generating a model containing a calculated total bitloading for an assumed gross coding gain and estimated numbers ofone-bit subchannels for a other plurality of assumed gross coding gainsand (2) loading bits in accordance with the model.

In still yet another aspect, the present invention provides a system forbit-loading in a DMT-based DSL modem. In one embodiment, the systemincludes: (1) a model generator configured to generate a modelcontaining a calculated total bit loading for an assumed gross codinggain and estimated numbers of occupied subchannels for a other pluralityof assumed gross coding gains and (2) a bit loader associated with themodel generator and configured to load bits in accordance with themodel. For purposes of the present invention, an “occupied subchannel”is a subchannel to which is allocated at least one bit.

In yet still another aspect, the present invention provides a method ofbit-loading in a DMT-based DSL modem. In one embodiment, the systemincludes: (1) generating a model containing a calculated total bitloading for an assumed gross coding gain and estimated numbers ofoccupied subchannels for a other plurality of assumed gross coding gainsand (2) loading bits in accordance with the model.

The foregoing has outlined preferred and alternative features of thepresent invention so that those skilled in the art may better understandthe detailed description of the invention that follows. Additionalfeatures of the invention will be described hereinafter that form thesubject of the claims of the invention. Those skilled in the art shouldappreciate that they can readily use the disclosed conception andspecific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a block diagram of a DSL line, a DMT-based DSL modemand one embodiment of an associated system for improving bit-loading inthe DMT-based DSL modem constructed according to the principles of thepresent invention; and

FIG. 2 illustrates a flow diagram of one embodiment of a method ofimproving bit-loading in a DMT-based DSL modem carried out according tothe principles of the present invention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, illustrated is a block diagram of a DSLline, a DMT-based DSL modem and one embodiment of an associated systemfor improving bit-loading in the DMT-based DSL modem constructedaccording to the principles of the present invention.

The DMT-based DSL modem, designated 100, is a rate-adaptive modem and iscoupled to a DSLAM 110 via a telephone line 120. The modem 100 isillustrated as including a model generator 130, bit loader 140, aprocessor 150 and a modulator/demodulator 160.

The model generator 130 is configured to generate a model. The modelcontains estimated numbers of occupied and bit-capped DMT subchannelsfor a other plurality of assumed gross coding gains and associatedassumed input parameters. From these estimated numbers, resultingestimated total bit loadings are calculated for the other plurality ofassumed gross coding gains and associated assumed input parameters. Anexemplary way in which the model generator 130 is configured to generatethis model will be set forth in greater detail in conjunction with FIG.2.

The bit loader 140 is associated with the model generator 130. The bitloader 140 is configured to load bits in accordance with the model so asto increase the overall net data rate of the DMT DSL modem 100. In theexemplary embodiment of FIG. 1, the model generator 130 takes the formof a sequence of software instructions executable in the processor 150.Of course, the model generator 130 may take the form of hardwiredcircuitry or firmware, as appropriate to a particular application. Asstated above, the bit loader 140 loads bits in accordance with themodel. In the specific embodiment of FIG. 1, the bit loader 140 loadsthe bits into a modulator portion of the modulator/demodulator 160 fortransmission over the telephone wire 120. Bits received from the DSLAM110 via the telephone wire 120 are demodulated in a demodulator portionof the modulator/demodulator 160 and delivered to the processor 150 forfurther use. Those skilled in the pertinent art are familiar with thestructure and operation of processors and modulator/demodulatorssuitable for use in a DMT DSL modem.

As stated above, the present invention deals with estimating the numberof bits loaded onto each DMT subchannel as the gross coding gainchanges. The only exact way to estimate this is to run an algorithmknown as “bit loading” over all DMT subchannels. Because bit loading iscomputationally intensive, the present invention provides a way to runbit loading only once at some initial gross coding gain but maintains anaccurate estimate of the bit loading over a range of gross coding gains.

The illustrated embodiment of the rate-adaptive DMT DSL modem 100 loopsthrough all possible subchannel coding gains to find the one that yieldsthe highest net data rate. The range of subchannel coding gains isapproximately 10 dB, and the step size used in the current version ofrate adaptation is such that many different subchannel coding gains aretested.

For each subchannel coding gain, the number of bits loaded per DMTsubchannel should be estimated under the assumption that increasing thegain increases the SNR. The number of DMT subchannels loaded with onlyone bit and those with more than one bit should also be estimated,because the number of trellis-coded bits is calculated differently forone-bit subchannels and subchannels with more than one bit.

One could in theory execute a bit loading algorithm (e.g., thewell-known Chow bit loading algorithm (see, Chow, et al., “A PracticalDiscrete Multitone Transceiver Loading Algorithm for Data Transmissionover Spectrally Shaped Subchannels,” IEEE Trans on Communications, Vol.43, No 2/3/4, pp. 773-775, February/March/April 1995, incorporatedherein by reference) for each new subchannel coding gain value. The bitloading algorithm would go through each subchannel and assign bitsaccording to the SNR level, margin and assumed subchannel coding gain.However, running the algorithm for each subchannel coding gain is notpractical given the constraints on computation time, especially in thedownstream direction, which has many subchannels.

Estimating the number of bits per subchannel is a reasonable way toaddress the problem. The bit loading algorithm could be run at a certainsubchannel coding gain and estimates for other additional bits at othersubchannel coding gains could be extrapolated.

For example, it is known that the number of bits loaded on eachsubchannel also increases by about one bit for every 3 dB as thesubchannel coding gain increases. However, the number of subchannelscarrying bits could increase. Furthermore, some subchannels could becomebit-capped or limited by the maximum number of bits per subchannelallowed by standard (typically 15 bits). The challenges facing thisapproach are that the 3 dB/bit/subchannel approximation does not holdvery well for small symbol constellations, estimating the number of newsubchannels is difficult, and keeping track of bit-capped subchannelscould mean going through each subchannel individually or estimating thisalso.

One possible implementation could estimate the number of bits andsubchannels at other gross coding gains after an initial bit loading atsome initial gross coding gain. The number of subchannels is estimatedby making some assumptions about the characteristics of the SNR and whatthe number of additional subchannels might be at a given increase insubchannel coding gain. Then the number of bits per symbol is estimatedby assuming 3 dB/bit/subchannel using the estimated number ofsubchannels.

In the upstream direction in ADSL Annex A and B, where only up to 26subchannels exist, this method will work well for a majority of cases.However, it is clear that if the SNR is not shaped in the way expected,the estimate will be inaccurate. If the SNR is flatter than expected, asmall increase in subchannel coding gain could result in a largeincrease in the number of supported subchannels. And, as the subchannelcoding gain grows further from the value used in the initial bitloading, the estimate becomes increasingly less accurate.

In the downstream direction, where up to 512 subchannels in ADSL2+ mayexist, this approximation will not generally be accurate. For subchannelcoding gain values far from the one used in the initial bit loading, theerror can be significant.

If the estimate of the number of bits loaded at a certain currentsubchannel coding gain is too high, then, in later stages of rateadaptation when the exact number of bits is loaded, the margin dropsbelow the targeted value. Similarly, if the number of bits is too low, alower data rate is being used than is achievable. Since operators testthe margin, any inaccuracy in this estimate is like a loss in SNR. Ifthe inaccuracy of the estimate normally changes the margin by ±1 dB, 1dB needs to be added to the design margin to ensure that the finalmargin does not drop to an unacceptable level.

Adding to the difficulty, the number of trellis-coded bits depends onthe number of one-bit constellations. Using a method of estimating thenumber of loaded subchannels, the number of one-bit subchannels can alsobe estimated. But this estimate will contain errors. If there are a lotof one-bit subchannels, the errors can be significant.

The present invention, in contrast, introduces a more accurateapproximation to the number of loaded bits and subchannels. Thisimproved approximation uses the exact number of loaded subchannels andaccounts for subchannels that are bit cap limited (for which the numberof bits would otherwise exceed the maximum bit count per subchannel, or“BIMAX”).

Based on the output of the single bit loading, three arrays arecomputed, indexed by the subchannel coding gain. The granularity of thesubchannel coding gain and the size of the arrays are defined constants.In the current code, the size of the three arrays is 192, and thegranularity is 16/256*10*log 10 (2)=0.1881 dB, making the total range ofthe arrays 192*0.1881≈36 dB. The three arrays indicate the number ofloaded subchannels, the number of one bit subchannels for use inestimating the number of trellis bits and the total number of loadedbits, all at the indexed gross coding gain.

Computing the three arrays requires one additional pass through allsubchannels (in addition to the first pass, which is a bit loadingalgorithm, such as the above-referenced Chow bit loading algorithm)followed by a computation stage that requires one pass through everyarray index. This computation is done after bitloading and beforelooping through all subchannel coding gains.

The illustrated embodiment of the algorithm of the present inventionbegins by computing three threshold values. The first two thresholdvalues are the minimum SNR the bitloading algorithm would use to loadBIMIN(1) bits and 2 bits (BIMIN being the minimum bit count persubchannel). Typically the threshold is about 1.5 dB below the SNRrequired to support exactly BIMIN or 2 bits, but the thresholds arecomputed in a table; the 1.5 dB approximation is not used.

The third threshold is exactly the SNR required to support BIMAX=15bits. The reason the threshold for BIMAX is at the SNR level required toload BIMAX bits, and not BIMAX+0.5 bits, will be explained shortly.

The SNR thresholds use pre-computed values that are based on thecalculated energy of each constellation. The desired margin and assumedsubchannel coding gain are included in the input variable “gapdB.”

With the three thresholds, the algorithm cycles through each subchannel.For each subchannel, the additional SNR required to reach the thresholdis calculated. In some cases, the additional SNR might be negative. Ifthe additional SNR is positive, with additional gain (or subchannelcoding gain) equal to the amount of additional SNR required, therespective number of bits could be loaded. The additional amount of gainis quantized to some index value. For example, if 4 dB additional gainis required to load 2 bits, assuming the granularity of indexed valuesis 0.1881 dB, the index would be equal to round(4/0.1881)=21.

Three temporary arrays are initialized to all zeros. At a given SNRindex value, the arrays represent the number of subchannels that: (1)start to have nonzero bits loaded (nloaded), (2) start to carry at least2 bits (nonebit) and (3) become bit cap limited (nnocap).

For example, if 4 dB additional gain is required to load two bitscorresponding with an index 21, 1 dB is required to load one bitcorresponding to an index 5, and 46 dB (index 390) is required to reachthe bit cap, then nloaded(5) would be incremented by one, since a 1 dBgain is required to load a bit onto a subchannel and nonebit(21) wouldbe incremented by one. nnocap would not change, since index 390 exceedsthe length of the arrays.

While looping through the subchannels, the number of subchannels, at theinitial subchannel coding gain level, that carry no bits, less than 2bits, and are not bit cap limited are tracked. So far, the complexity ofthe algorithm is on the order of the number of subchannels (similar tobit loading itself).

With the arrays, a new set of arrays can be computed that, at a givensubchannel coding gain level, contain: (1) the number of subchannelswith bits loaded (nloaded), (2) the number of subchannels with one bitloaded (nonebit) and (3) the number of subchannels that are notbit-capped but carry bits (nnocap).

The same arrays are used to store this information to save storagespace. Each array is initialized as: (1) loaded(0)=the total number ofsubchannels minus the number of subchannels with zero bits, (2)nonebit(0)=the number of subchannels with less than two bits minus thenumber of subchannels with zero bits and (3) nnocap(0)=the number ofsubchannels that are not bit cap limited minus the number of subchannelswith zero bits.

Then a loop over the length of the array is executed, noting that:nonebit(i)=nonebit(i−1)+nloaded(i)−nonebit(i)   (1)nnocap(i)=nnocap(i−1)+nnocap(i)+nloaded(i)   (2)nloaded(i)=nloaded(i−1)+nloaded(i)   (3)

Equation (1) states that the number of subchannels with loaded bits atindex i is the number of subchannels with one bit at index i−1 plus thenumber of subchannels that have one bit loaded at index i minus thenumber of subchannels that no longer have one bit loaded at this highergain. Equations (2) and (3) are similar in structure. Note thatnonebit(i) on the right side of the equation is the old value, while thenew value is on the left side.

Finally, a second loop over the array length is performed to find thenumber of loaded bits at a given subchannel coding gain. A 3dB/bit/subchannel approximation is used. It is found that the number ofloaded bits at index i is the number of loaded bits at indexi−1+nloaded(i)*0.1881/3. The 3 dB/bit/subchannel approximation meansthat the estimated number of loaded bits loses some accuracy when manysubchannels are loaded with 1, 2 and 3 bits. The second two loops are onthe order of the array size in complexity.

The number of loaded subchannels and the number of one bit subchannelsare not approximations; they correspond exactly to what bit loadingwould give at the same gross coding gain. The number of bits loaded at agiven subchannel coding gain is, however, an approximation based on the3 dB/bit/subchannel approximation. But since the number of loadedsubchannels is exactly what full bit loading would predict, theestimated number of loaded bits will be very accurate.

The size of the arrays is influenced not only by the possible range ofgross coding gain, but also by the desire to reduce excess margin. Onshort channels with high SNR, even at the highest rates, the margin canbe well in excess of the required margin. In this case, it is desirableto reduce the transmit power. The amount by which the transmit power isreduced can be determined using the arrays.

Since the illustrated embodiment of the algorithm of the presentinvention is referenced to the last time bit loading was called, whenthe arrays are indexed by 0 (zero), this is normally the number of bitsand subchannels that were loaded during bit loading. However, sometimesit is useful to estimate the number of bits at a subchannel coding gainthat is below the level used during the previous bit loading. For thisreason, an “offset” is included in the algorithm. Instead of 0, when thearrays are indexed by the offset, this is the reference subchannelcoding gain used by the previous bit loading. If the offset is greaterthan 0, the index 0 is actually an estimate of the bits and subchannelsloaded at a subchannel coding gain that is offset lower than the oneused during the previous bit loading.

Turning now to FIG. 2, illustrated is a flow diagram of one embodimentof a method of improving bit-loading in a DMT-based DSL modem carriedout according to the principles of the present invention.

The method begins in a start step 210 in which it is desired to improvebit-loading. The method proceeds to a step 220 in which a table iscreated. In one embodiment of the present invention, three tables arebuilt, indexed by gross coding gain. Of course, methods that do notinvolve tables fall within the broad scope of the present invention.

Next, in a step 230, occupied and bit-capped DMT subchannels arecalculated for that gross coding gain. Step 230 may be performed by aconventional bit loading algorithm, such as the Chow algorithmreferenced above. Then in a step 240, resulting total bit loadings foreach gross coding gain are calculated. In this manner, a suitable modelis generated for use.

Next, in a step 250, actual gross coding gain is determined underoperating conditions (typically by monitoring subchannel SNR). Then in astep 260, the model is consulted to determine the bit loadingcorresponding to that actual gross coding gain that offers a superior(and most preferably maximum) net data rate. In a step 270, that bitloading is employed to load bits. The method end in an end step 280.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

1. A system for bit-loading in a discrete multitone (DMT)-based digitalsubscriber line (DSL) modem, comprising: a model generator configured togenerate a model containing a calculated total bit loading for anassumed gross coding gain and estimated total bit loadings for aplurality of other assumed gross coding gains; and a bit loaderassociated with said model generator and configured to load bits inaccordance with said model.
 2. The system as recited in claim 1 whereinsaid model generator estimates said estimated total bit loadings usingan array subscripted by said assumed gross coding gains.
 3. The systemas recited in claim 1 wherein said model generator estimates saidestimated total bit loadings by adding total numbers of estimated bitsloaded on all subchannels at said assumed gross coding gains.
 4. Thesystem as recited in claim 3 wherein said total numbers of estimatedbits loaded on all subchannels depends upon signal-to-noise ratios, bitcaps and numbers of unloaded subchannels.
 5. A method of bit-loading ina discrete multitone (DMT)-based digital subscriber line (DSL) modem,comprising: generating a model containing a calculated total bit loadingfor an assumed gross coding gain and estimated total bit loadings for aother plurality of assumed gross coding gains; and loading bits inaccordance with said model.
 6. The method as recited in claim 5 whereinsaid generating comprises estimating said estimated total bit loadingsusing an array subscripted by said assumed gross coding gains.
 7. Themethod as recited in claim 5 wherein said generating comprisesestimating said estimated total bit loadings by adding total numbers ofestimated bits loaded on all subchannels at said assumed gross codinggains.
 8. The method as recited in claim 7 wherein said total numbers ofestimated bits loaded on all subchannels depends upon signal-to-noiseratios, bit caps and numbers of unloaded subchannels.
 9. A system forbit-loading in a discrete multitone (DMT)-based digital subscriber line(DSL) modem, comprising: a model generator configured to generate amodel containing a calculated total bit loading for an assumed grosscoding gain and estimated numbers of one-bit subchannels for a otherplurality of assumed gross coding gains; and a bit loader associatedwith said model generator and configured to load bits in accordance withsaid model.
 10. The system as recited in claim 9 wherein said modelgenerator estimates said estimated numbers of one-bit subchannels usingan array subscripted by said assumed gross coding gains.
 11. The systemas recited in claim 9 wherein said model generator estimates saidestimated numbers of one-bit subchannels by adding total numbers ofone-bit subchannels at said assumed gross coding gains.
 12. The systemas recited in claim 11 wherein said numbers of one-bit subchannelsdepend upon signal-to-noise ratios, bit caps and numbers of unloadedsubchannels.
 13. A method of bit-loading in a discrete multitone(DMT)-based digital subscriber line (DSL) modem, comprising: generatinga model containing a calculated total bit loading for an assumed grosscoding gain and estimated numbers of one-bit subchannels for a otherplurality of assumed gross coding gains; and loading bits in accordancewith said model.
 14. The method as recited in claim 13 wherein saidgenerating comprises estimating said estimated numbers of one-bitsubchannels using an array subscripted by said assumed gross codinggains.
 15. The method as recited in claim 13 wherein said generatingcomprises estimating said estimated numbers of one-bit subchannels byadding total numbers of one-bit subchannels at said assumed gross codinggains.
 16. The method as recited in claim 15 wherein said numbers ofone-bit subchannels depend upon signal-to-noise ratios, bit caps andnumbers of unloaded subchannels.
 17. A system for bit-loading in adiscrete multitone (DMT)-based digital subscriber line (DSL) modem,comprising: a model generator configured to generate a model containinga calculated total bit loading for an assumed gross coding gain andestimated numbers of occupied subchannels for a other plurality ofassumed gross coding gains; and a bit loader associated with said modelgenerator and configured to load bits in accordance with said model. 18.The system as recited in claim 17 wherein said model generator estimatessaid estimated numbers of occupied subchannels using an arraysubscripted by said assumed gross coding gains.
 19. The system asrecited in claim 17 wherein said model generator estimates saidestimated numbers of occupied subchannels by adding total numbers ofoccupied subchannels at said assumed gross coding gains.
 20. The systemas recited in claim 19 wherein said numbers of occupied subchannelsdepend upon signal-to-noise ratios, bit caps and numbers of unloadedsubchannels.
 21. A method of bit-loading in a discrete multitone(DMT)-based digital subscriber line (DSL) modem, comprising: generatinga model containing a calculated total bit loading for an assumed grosscoding gain and estimated numbers of occupied subchannels for a otherplurality of assumed gross coding gains; and loading bits in accordancewith said model.
 22. The method as recited in claim 21 wherein saidgenerating comprises estimating said estimated numbers of occupiedsubchannels using an array subscripted by said assumed gross codinggains.
 23. The method as recited in claim 21 wherein said generatingcomprises estimating said estimated numbers of occupied subchannels byadding total numbers of occupied subchannels at said assumed grosscoding gains.
 24. The method as recited in claim 23 wherein said numbersof occupied subchannels depend upon signal-to-noise ratios, bit caps andnumbers of unloaded subchannels.